An analog-to-digital conversion device receives an analog input signal and converts it into a corresponding digital output signal having a predetermined number of digital bits. The accuracy of such a conversion device is normally rated in terms of a percentage of the full-scale of the input signal.
In many instances it is important to know the accuracy of the digital output signal relative to the actual value of the input signal, not the nominal full-scale value of the input signal. A specific example is an A/D converter which has a full-scale input signal of ten (10) volts and an actual input signal of one (1) volt, with a required accuracy of one percent (1%) relative to the actual input value. In this case, the output value must be accurate to within 10 millivolts to satisfy the requirement of accuracy with respect to the actual input value. If the converter were one percent (1%) accurate relative to the full-scale input value, the accuracy of the output would only be within 100 millivolts.
There are a number of engineering applications where it is desirable that the output of the converter be accurate relative to the actual input value, not the full-scale input value. An example is in an automotive fuel injection system where the fuel-to-air ratio of the injected mixture must be accurate to within a precise tolerance. The fuel injected is based upon a measurement of the air flow, and this air flow must be accurately conveyed to the fuel control computer. A deviation beyond this precise tolerance can cause a marked falloff in fuel efficiency and diminish the effectiveness of anti-pollution equipment.
An objective, therefore, of the present invention is to provide conversion devices and associated methods of operation that perform a conversion relative to the actual value of the input signal, and not the full-scale value. In particular, the object of the invention is an analog-to-digital converter and its compansion digital-to-analog converter accurate relative to the actual value of the received signal.